Vignettes > Megathrust earthquakes, coastal uplift, and emergent marine terraces of Costa Rica’s Nicoya Peninsula

Megathrust earthquakes, coastal uplift, and emergent marine terraces of Costa Rica's Nicoya Peninsula

Jeff Marshall Cal Poly Pomona University

Description Location Megathrust earthquakes along subduction zones are among Continent: Central America Earth's most powerful and deadly natural hazards. Over a Country: Costa Rica quarter of a million people were killed by the 2004 Indian State/Province: Nicoya Peninsula Ocean tsunami generated by the Sumatra-Andaman City/Town: megathrust earthquake. Such catastrophic events are also UTM coordinates and datum: none notable for the sudden geomorphic change that they bring to coastlines through coseismic uplift and subsidence (e.g., Setting Plafker, 1972). Understanding how these sudden changes Climate Setting: Tropical affect the long-term growth and decay of coastal topography Tectonic setting: Continental Arc is an intriguing puzzle for geomorphologists. Type:

An excellent place to study this process is the Nicoya Peninsula on Costa Rica's Pacific coast in Central America (Fig. Click the images for a full-sized view. 1). The Nicoya Peninsula is unique because it is one of the few landmasses along the Pacific Rim located directly above the seismogenic zone of a subduction megathrust. Due to its proximity to the subduction trench, the Nicoya Peninsula is particularly sensitive to vertical movements related to the earthquake cycle.

Costa Rica is part of the Central American volcanic front, formed by subduction of the Cocos tectonic plate beneath the Caribbean plate at the Middle America Trench (Fig. 2). The two plates converge at a rapid rate (~9 cm/yr), resulting in a high seismic potential, as demonstrated by multiple large magnitude (>M 7.0) earthquakes during the past few centuries (Fig. 3). The last major earthquake centered beneath the Nicoya Peninsula occurred on October 5, 1950 with a magnitude of Mw 7.7. This event killed and injured dozens of people, severely damaged buildings and roads, and

produced landslides, liquefaction, and coseismic coastal uplift Digital elevation model of southern Central (Marshall and Anderson, 1995). Based on seismologic and America, showing NASA-SRTM topography for Nicaragua and Costa Rica linked to offshore geodetic studies, the Nicoya Peninsula is now recognized as a IFM-GEOMAR multi-beam bathymetry (courtesy of C. Ranero, ICM-CSIC, Barcelona; mature seismic gap, with the next large earthquake expected after Ranero et al., 2008). This image in the near future (Protti et al., 2001). In 1950, the Nicoya

1 of 5 coast was a sparsely populated frontier with thick rainforests reveals the relationship between the morphology of the subducting Cocos Plate and isolated rural villages. In recent years, however, this and the morphotectonic structure of the overriding fore arc and volcanic arc of the beautiful region has become an epicenter for rapid coastal Caribbean Plate and Panama block. The development related to Costa Rica's world-renowned tourism Nicoya Peninsula lies along the Middle America Trench at the center of the image. trade. Construction of hotels, condominiums, and vacation Courtesy of C. Ranero, ICM-CSIC, Barcelona, Spain. homes proceeds without heed for the lurking earthquake hazard. It is critical, therefore, that geoscientists, government officials, and local residents develop a better understanding of the megathrust earthquake cycle beneath the Nicoya Peninsula.

The following story provides a graphic account of one man's experience during the 1950 Nicoya Peninsula earthquake. His recollection of dramatic coastal changes is similar to those of other earthquake survivors interviewed by geoscientists studying seismic cycle deformation patterns (Marshall, 1991). Digital elevation model of Central America and adjacent sea floor of the Cocos, Nazca, The sky dawned dark and cloudy on the morning of October and Caribbean plates (IFM-GEOMAR). The Middle America Trench marks the convergent 5, 1950. It was rainy season along the Pacific coast of Costa margin where the Cocos Plate subducts Rica's Nicoya Peninsula. In scattered coastal villages, farmers northeastward at 8-10 cm/yr beneath the Caribbean Plate and Central American and ranchers gathered along the shoreline waiting for the volcanic front. This image reveals a sharp contrast on the Cocos Plate between smooth cargo launch to arrive from the port city of Puntarenas. Their sea floor formed at the East Pacific Rise (EPR) ox-carts were full of freshly harvested corn, ready for and rough, hotspot-thickened sea floor formed along the Cocos-Nazca spreading shipment to the mainland. Near the village of Garza, Don center (CNS). This boundary intersects the Middle America Trench offshore of Costa Daniel Ruíz Matarita was riding on horseback along the beach Rica's Nicoya Peninsula. The rectangle with several other men. The horses were skittish, and the outlines the area of Costa Rica and Nicaragua shown in Figure 1. By Jeff Marshall nearby rainforest echoed with the roar of nervous howler (background DEM courtesy of C. Ranero, monkeys. Suddenly, without warning, the ground heaved ICM-CSIC, Barcelona, Spain). violently, trees and branches toppled over, and the beach exploded with geysers of water and sand. "Earthquake! We're done for!" shouted one of the men. Their horses bolted in terror, throwing the riders to the ground. Huge chunks of rock toppled off of nearby cliffs, crashing into the water with a tremendous splash. Certain they were doomed, the men prayed for salvation. When the great earthquake finally subsided, Don Daniel and his companions were amazed and thankful to be alive. As they stood up and looked around, they saw that the ocean curiously had withdrawn from the bay, leaving a wide expanse of barren rocks, seaweed, and flopping fish. Seizing the moment, the men snatched up handfuls of sea bass, content at least that their bellies would be full in this time of disaster. In the days following the earthquake, Don Daniel recalls that the sea did not return as they had expected. He heard stories from others that the same thing had happened all along the central Nicoya coast. Geologic map of the Nicoya Peninsula showing boundaries of the Nicoya seismic Don Daniel remembers one place where the drop in sea level gap (orange dashed lines), epicenters of was particularly obvious, a rocky headland known to local large subduction earthquakes (red circles), and aftershock zones of the 1950, 1978, fisherman as "La Raspa Nalgas" (The Butt Scratcher). Prior to 1990, and 1992 events (red dashed lines). The limits of the 1990 and 1992 ruptures the earthquake, it had been impossible to get around this coincide with the edges of the seismic gap. rocky point on foot, as it was under water at even the lowest The last major rupture of the Nicoya segment (1950; M=7.7) produced 1-2 m of coseismic tides. But, after the quake one could walk around the coastal uplift (Marshall and Anderson, 1995). By Jeff Marshall (earthquake data from Protti headland without entering the water, indicating a drop in et al., 2001). tidal levels near a grown man's height. Don Daniel recalls that

2 of 5 it took nearly four decades for the ocean to reclaim its former level, quickly during the first few years, then slowly thereafter. High tides now reach further inland in many places than they did before the 1950 earthquake.

Such stories provide compelling evidence for abrupt coseismic uplift followed by gradual postseismic and interseismic subsidence. The return of tides to their Digital elevation model of Costa Rica (SRTM) pre-1950 levels indicates that the Nicoya Peninsula is locked linked to offshore bathymetric data (courtesy and loaded for the next major earthquake. Similar of C.J. Petersen, IFM-GEOMAR). Two segment boundaries on the subducting Cocos Plate movements have been observed along convergent margin (Barckhausen et al., 2001) intersect the margin offshore of the Nicoya Peninsula: 1) a coastlines worldwide. Famous examples in the geomorphic morphologic break between smooth and literature include Chile, Alaska, Japan, Cascadia, Vanuatu, and rough sea floor domains (thin dashed line); and 2) a fracture zone trace (thick dashed Indonesia (Plafker, 1972; Matsuda et al., 1978; Bull, 1985; line) that divides crust formed at the East Pacific Rise (EPR) from that formed at the LaJoie, 1986; Atwater, 1987; Taylor et al., 1987; Sieh, Cocos-Nazca spreading center (CNS-1 and 2). 2006). As the locked interface between two converging The southern edge of the Nicoya Peninsula coincides with the Central Costa Rica tectonic plates snaps free, the upper plate springs forward deformed belt (red dashed line), a diffuse transpressional fault zone between the releasing stored elastic energy in the form of seismic waves Caribbean Plate and Panama block inboard (the earthquake). The seaward edge of the plate nearest the of the subducting Cocos Ridge (Marshall et al., 2000). Rectangle outlines the Nicoya subduction trench rebounds upward, resulting in sudden Peninsula as shown in Fig. 5. By Jeff Marshall (background DEM courtesy of C.J. Petersen, coseismic uplift (and often a tsunami). In contrast, the IFM-GEOMAR, Germany). landward region further from the trench subsides as strain is released. As the plates become locked again, and elastic strain begins to build, gradual interseismic movements generally occur in the opposite direction (subsidence in the cosesimic uplift zone and vice versa). This cycle of vertical motion in response to elastic strain accumulation and release is an integral part of the way subduction zones work.

An interesting question for geomorphologists is how this short-term cycle of elastic motion translates into longer-term permanent deformation that generates topographic relief. How much of the seismic cycle deformation is non-recoverable and permanent? Does coastal topography mirror earthquake cycle deformation patterns? Are earthquake rupture zones long-lived features or are they

transient, changing location and shape through time? What Upper Image: Digital elevation model of the other processes contribute to the creation of permanent Nicoya Peninsula (NASA-SRTM) showing areas of uplifted marine and fluvial terraces coastal topography along convergent margins? within the late Pleistocene Iguanazul, Carrillo-Camaronal, Cobano, and La Mansión geomorphic surfaces (red areas); and the Along the Nicoya Peninsula's seaward-facing coastline (Fig. Holocene Garza and Cabuya surfaces (yellow areas). Rectangles outline the three coastal 4), net Quaternary uplift is recorded by emergent marine study areas shown in Figs. 6a-c. Lower terraces (ancient shorelines) and uplifted alluvial fill (ancient Image: Summary diagram showing uplifted terrace elevations, age data, and uplift rates river deposits) (Hare and Gardner, 1985; Marshall and within each coastal study area. Numbers in circles indicate specific study sites for Anderson, 1995; Gardner et al., 2001; Marshall et al, 2001). Pleistocene and Holocene terraces. Terrace Along the peninsula's landward-facing gulf coast, net ages based on sea level curve correlations and isotopic dates (OSL and 14C, as subsidence results in drowned rivers and broad mangrove indicated). Differences in uplift rates for each study area reflect variations in the estuaries. Ongoing geomorphic, paleo-geodetic, and subducting seafloor and seismogenic zone paleoseismic studies (e.g., Marshall et al., 2007) are revealing structure across the EPR, CNS-1, and CNS-2 segment boundaries. By Jeff Marshall upper plate deformation patterns that provide important (background image courtesy of NASA-JPL). clues about seismogenic zone segmentation and the periodicity of megathrust earthquakes beneath the Nicoya Peninsula. Field mapping, surveying, and isotopic dating of uplifted paleo-shorelines, river deposits, and wetland sediments allows for calculation of Holocene and Pleistocene uplift rates. Preliminary

3 of 5 results (Fig. 5; Marshall et al., 2007) indicate that sharp variations in uplift patterns on the Nicoya Peninsula coincide with three distinct domains of subducting seafloor identified through offshore geophysical studies. These seafloor segments (Fig. 4), designated EPR, CNS-1, and CNS-2 (Barckhausen et al., 2001), each originated at distinct oceanic spreading ridges and exhibit contrasts in crustal thickness, surface roughness, and heat flow (e.g., von Huene et al., 2000; Fisher et al., 2003). Such contrasts may exert important controls on seismogenic zone geometry, seismic coupling, and earthquake rupture behavior (e.g., Newman et al., 2002; Norabuena et al., 2004; DeShon et al., 2006; Schwartz and DeShon, 2007).

Field based geomorphic research, in conjunction with instrumental seismology, geodesy, and marine geophysical studies (see NSF MARGINS Program web site), are helping to advance our understanding of megathrust earthquake hazards along the Nicoya Peninsula seismic gap. While the recollections of Don Daniel Ruíz Matarita and his neighbors provide us with a dramatic window into the human experience of a major Nicoya Peninsula earthquake, geoscientists are now helping elucidate a scientific picture of the natural processes behind such events.

Associated References: Atwater, B., 1987, Evidence of great Holocene earthquakes along the outer coast of Washington state, Science, v. 236, 942-944. Barckhausen, U., Ranero, C.R., von Huene, R., Cande, S.C., and Roeser, H.A., 2001, Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models: Journal of Geophysical Research, v. 106, p. 19,207–19,220. Bull, W.B., 1985, Correlation of flights of global marine terraces, in Morisawa, M., and Hack, J.T., eds., Tectonic Geomorphology: Proceedings of the 15th Geomorphology Symposia Series, Binghamton, p. 129-152. DeShon, H.R., Schwartz, S.Y., Newman, A.V., González, V., Protti, M., Dorman, L.M., Dixon, T.H., Sampson, D.E., and Flueh, E.R., 2006, Seismogenic zone structure beneath the Nicoya Peninsula, Costa Rica, from three-dimensional local earthquake P- and S-wave tomography: Geophysical Journal International, v. 164, p. 109–124. Fisher, A.T., Stein, C.A., Harris, R.N., Wang, K., Silver, E.A., Pfender, M., Hutnak, M., Cherkaoui, A., Bodzin, R. and Villinger, H., 2003, Abrupt thermal transition reveals hydrothermal boundary and role of seamounts within the Cocos Plate: Geophysical Research Letters, v. 30, 1550, doi:10.1029/2002GL016766. Gardner, T.W., Marshall, J.S., Merritts, D.J., Protti, M., Bee, B., Burgettte, R., Burton, E., Cooke, J., Kehrwald, N., Fisher, D., and Sak, P., 2001, Holocene forearc block rotation in response to seamount subduction, southeastern PenÌnsula de Nicoya, Costa Rica: Geology, v. 29, p. 151-154. Hare, P.W., and Gardner, T.W., 1985, Geomorphic indicators of vertical neotectonism along converging plate margins, Nicoya Peninsula, Costa Rica, in Morisawa, M., and Hack, J.T., eds., Tectonic geomorphology: Proceedings of the 15th Geomorphology Symposia Series, Binghamton, p. 76-104. LaJoie, K.R., 1986, Coastal Tectonics, in R.E. Wallace, chairman, Active Tectonics, Studies in Geophysics, 95-124, National Academy Press, Washington, D.C. Marshall, J.S., 1991, Neotectonics of the Nicoya Peninsula, Costa Rica: A Look at Forearc Response to Subduction at the Middle America Trench: M.S. Thesis, University of California Santa Cruz, 196 p. Marshall, J.S., and Anderson, R.S., 1995, Quaternary uplift and seismic cycle deformation, Península de Nicoya, Costa Rica: Geological Society of America Bulletin, v. 107, p. 463-473. Marshall, J.S., Gardner, T.W., Fisher, D.M., Sak, P.B. and Protti, M., 2001, Quaternary

4 of 5 neotectonics of the Costa Rican coastal fore arc, Field Trip Guide, National Science Foundation MARGINS Program: Central America Tectonics Workshop, Heredia, Costa Rica, July 2001, 62 p. Marshall, J.S., LaFromboise, E.J., Gardner, T.W., and Protti, M., 2007, Segmented fore arc deformation along the Nicoya Peninsula seismic gap, Costa Rica: Eos, Transactions, American Geophysical Union, v. 88, Fall Meeting Supplement, Abs. T53A-1121. Matsuda, T., Ota, Y., Ando, M., and Yonekura, N., 1978, Fault mechanism and recurrence time of major earthquakes in Southern Kanto district, Japan, as deduced from coastal terrace data: Geological Society of America Bulletin, v. 89, p. 1610–1628. Newman, A.V., Schwartz, S.Y., Gonzales, V., DeShon, H.R., Protti, J.M., and Dorman, L., 2002. Along strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica, Geophysical Research Letters, v. 29, doi: 10.1029/2002GL015409. Norabuena, E., Dixon, T.H., Schwartz, S., DeShon, H., Newman, A., Protti, A., González, V., Dorman, L., Flueh, E.R., Lundgren, P., Pollitz, F., and Sampson, D., 2004, Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica: Journal of Geophysical Research, v. 109, B11403, doi: 10.1029/2003JB002931. Plafker, G., 1972, The Alaskan earthquake of 1964 and Chilean earthquake of 1960; Implications for arc tectonics and tsunami generation: Journal of Geophysical Research, v. 77, p. 901-925. Protti, M., Güendel, F., and Malavassi, E., 2001, Evaluación del potencial sísmico de la Península de Nicoya, Heredia, Costa Rica, Editorial Fundación UNA, 144 p. Ranero, C.R., Grevemeyer, I., Sahling, H., Barckhausen, U., Hensen, C., Wallmann, K., Weinrebe, W., Vannucchi, P., von Huene, R., and McIntosh, K., 2008, Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis: Geochemistry, Geophysics, Geosystems, v. 9, Q03S04, doi:10.1029/2007GC001679. Schwartz, S.Y., and DeShon, H.R., 2007, Distinct updip limits to geodetic locking and microseismicity at the northern Costa Rica seismogenic zone: Evidence for two mechanical transitions, in The Seismogenic Zone of Subduction Thrust Faults, Dixon, T., and Moore, J.C., eds., Columbia Univerisity Press, NewYork, p. 576-599. Sieh, K., 2006, Sumatran Megathrust Earthquakes - From Science to Saving Lives: Philosophical Transactions of the Royal Society, v. 364 (1845), p. 1947-1963. Taylor, F.W., Frohlich, C., Lecolle, J., Strecker, M., 1987, Analysis of partially emerged corals and reef terraces in the central Vanuatu arc—comparison of contemporary coseismic and nonseismic with Quaternary vertical movements: Journal of Geophysical Research-Solid Earth, v. 92 (B6), 4905–4933. von Huene, R., Ranero, C., Weinrebe, W., and Hinz, K., 2000, Quaternary convergent margin tectonics of Costa Rica: Segmentation of the Cocos Plate and Central American volcanism: Tectonics, v.19, p. 314-334.

Supporting URLs Jeff Marshall's Nicoya Peninsula Research Home Page MARGINS: SEIZE and SubFac Focus Site: Costa Rica-Nicaragua Obvservatorio Vulcanológico y Sismológico de Costa Rica

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